Alike to r242314 for GRAID make GMIRROR more aggressive in marking volumes

Alike to r242314 for GRAID make GMIRROR more aggressive in marking volumes
as clean on shutdown and move that action from shutdown_pre_sync stage to
shutdown_post_sync to avoid extra flapping.

ZFS tends to not close devices on shutdown, that doesn't allow GEOM RAID
to shutdown gracefully.  To handle that, mark volume as clean just when
shutdown time comes and there are no active writes.

PR:		kern/113957
MFC after:	2 weeks